Spider-Man is suspended in the air in a scene from the musical "Spider-Man: Turn Off the Dark." Photo: AP

“Spider-Man, Turn Off the Dark’’ is not only the most expensive show in Broadway history – it’s the most dangerous.

A third actor was injured during the show this week.

Natalie Mendoza, who plays Arachne, was hit on the head by a rope at the first preview Sunday night. Although she felt fine and performed again Wednesday night, she fell ill yesterday and could not perform last night. Her absence was first reported by BroadwayWorld.com.

The actress was diagnosed with a concussion and will be out of the show at least a week.

Her understudy, America Olivo, a former Playboy cover model, will take over the role.

This is the third accident to beset the troubled show which is due to open Jan. 11. Kevin Aubin, an aerialist, broke both his wrists last month during a presentation for ticket agents. In a separate incident while rehearsing the show, another actor broke his foot.

The show’s producers were not immediately available for comment.

Calls to Actors Equity were not immediately returned. A source close to the union, which represents the actors and looks after their safety on Broadway, told The Post, “I’m very disappointed in Equity. They seem to have been somewhat lax about safety from the beginning. They should have been in there before any of these injuries happened.’’